1. Field of the Invention
The present invention relates to a package for a surface mount device (SMD) in which an electronic device is encapsulated in a cavity formed between two substrates bonded together, and more particularly, to a structure to bond two substrates.
2. Background Art
Recently, electronic devices using a compact surface mount device package are employed often in mobile phones and personal digital assistants. Of these electronic devices, many components, such as a transducer, an MEMS, a gyrosensor, and an acceleration sensor, require a package of a hollow cavity structure. A structure in which a base substrate and a cover substrate are boned together via a metal film is known as a package of the hollow cavity structure. Also, eutectic bonding, seam bonding, and anodic bonding are known as a bonding method. Details are described, for example, in JP-A-09-002845.
A manufacturing method of a package in the related in which a base substrate and a cover substrate are bonded together via a metal film will now be described. In particular, a description will be given to a manufacturing method by which a plurality of package elements are formed in array on a single sheet of base substrate and after a cover substrate is boned to the base substrate, the bonded substrates are divided into individual packages.
As are shown in FIG. 7A through FIG. 7E, an electronic device package in the related art includes an electronic device 47, a plate-shaped base substrate 41, a cover substrate 42 provided with a concave portion, and a metal film 49, which is a bonding film to bond the base substrate 41 and the cover substrate 42 together. Because the cover substrate 42 is provided with a concave portion, a cavity 46 is formed by sealing the base substrate 41 with the cover substrate 42. The electronic device 47 is accommodated in the cavity 46.
The base substrate 41 is formed of an insulator, a semiconductor, or metal and formed in a plate shape. Wires 43 used to mount the electronic devices 47 are formed on the surface of the base substrate 41 in a number according to the number of the electronic devices 47 to be mounted. Outside electrodes 45 are formed on the back surface of the base substrate 41 in a corresponding manner to the wires 43. In order to connect the wires 43 on the front surface of the base substrate 41 and the corresponding outside electrodes 45 on the back surface, through-holes are formed at arbitrary portions of the packages and feed-through electrodes 44 are formed to fill the respective through-holes. The wires 43 and the outside electrodes 45 are thus connected via the feed-through electrodes 44.
The cover substrate 42 is formed of an insulator, a semiconductor, or metal and formed to have concave portions. When the cavities 46 are formed as the base substrate 41 and the cover substrate 42 are bonded together, the metal film 49 is formed as the bonding film in a portion where the base substrate 41 and the cover substrate 42 come into contact with each other. Basically, it is sufficient to form the metal film 49 only on the portion where the base substrate 41 and the cover substrate 42 come into contact with each other. However, by taking simplification of the steps into account, as is shown in FIG. 7B, the metal film 49 is formed entirely on one surface of the cover substrate 42.
The manufacturing method will now be described. A plurality of concave cavities 46 are formed in a wafer of cover substrate 42 so that a plurality of electronic devices 47 can be mounted thereon (FIG. 7A). Thereafter, the metal film 49 as a bonding film is formed on one surface of the cover substrate 42 (FIG. 7B). Aluminum, chrome, silicon, and copper are suitable as the metal film 49. The wires 43 used to mount a plurality of the electronic devices 47 on a wafer of base substrate 41, the outside electrodes 45, and the feed-through electrodes 44 are formed (FIG. 7C). Subsequently, the electronic devices 47 are mounted on the base substrate 41 and the electronic devices 47 and the wires 43 are connected with wires 48 by wire bonding (FIG. 7D).
The base substrate 41 and the cover substrate 42 are aligned and superimposed, and then bonded together. When bonded together, as is shown in FIG. 7E, the base substrate 41 and the cover substrate 42 aligned with each other are sandwiched by substrates 50 and 51 serving as heaters and also as electrodes. A positive electrode probe 52 is set so as to come into contact with the metal film 49 and temperatures of the substrates 50 and 51 serving as heaters and also as electrodes are raised. A voltage is then applied between the positive electrode probe 52 and the substrate 50 serving as a heater and also as an electrode. Consequently, the base substrate 41 and the cover substrate 42 are bonded together via the metal film 49. Thereafter, package elements are cut off individually using a dicing apparatus or the like. Individual electronic device packages are thus completed.
The manufacturing method of the electronic device package in the related art, however, has problems as follows. Firstly, because a plurality of concave portions that later form the cavities 46 are formed in one surface of the cover substrate 42, when the cover substrate 42 is made thinner, shapes of one surface and the other surface of the cover substrate 42 differ from each other. This poses a problem that the cover substrate 42 warps considerably. To eliminate this problem, a countermeasure to suppress warping is taken by increasing a film thickness of the metal film 49 formed on the cover substrate 42 on the surface in which the concave portions are formed. However, when the metal film 49 becomes thicker, there arises another problem that bonding strength between the metal film 49 and the base substrate 41 becomes lower.